[XeTeX] suppress natbib warnings

[maxwell]

I'm trying to explain to one of our people how to track down missing
citations.  We're using the natbib package, and it outputs warnings like:
   Package natbib Warning: Citation `Maloney1980' on page 1 
     undefined on input line 211.
We run the typical four passes, and redirect all these warnings etc. to a
file.  The problem is that there is a ton of extraneous warnings from the
first couple passes--the only warnings we want to worry about are those in
the last pass.  

In order to make it easier to find the relevant warnings (and only the
relevant ones), I'd like to suppress such warnings on the first couple
passes.  One obvious way would be to redirect xelatex's output to /dev/null
on the first couple passes, except that if it completely crashes on that
pass, we need to know why.  So that won't work.

I tried the command line parameter -no-file-line-error, but that doesn't
have the desired effect.  (I can't tell that it has *any* effect, but maybe
that's just me.)

I tried piping xetex's output through 
   grep -v natbib
on the first couple passes.  That eliminates the first line of the natbib
warning msgs, but if the line gets wrapped (why is it wrapped in output
piped to a file?), the second line is still there--things like
      undefined on input line 211

Is there no way to tell xetex to suppress warnings, while keeping true
error msgs?

   Mike Maxwell